Laser machining of silicon with bursts of ultra-short laser pulses: Factors influencing the process efficiency and surface quality (Conference Presentation)

For silicon machined with 10 ps pulses at 1064nm it was found that the specific removal rate increases by a factor of about 2.5 when an 8-pulse burst is used instead of single pulses [1]. This increase in the specific removal rate directly scales with a higher surface roughness. For copper and a 3-pulse burst the absorptance of a machined surface increases to about 200% of the one obtained with single pulses [2]. This can serve as an explanation for the higher specific removal rate observed in this case [3]. Actual calorimetric measurements on silicon show that the fraction of the incoming energy which is converted to heat is almost independent on the number of pulses per burst (a behavior which was observed for copper too) and the absorptance depends on the number of pulses. However, as the observed variations in the absorptance are only in the order of a few % and do not monotonically increase with the number of pulses per burst the change in the absorptance cannot explain the increase in the specific removal rate for pulse bursts on silicon. Additional experiments including calorimetry with varying intra-burst time differences and pulse energies as well as experiments concerning reflectivity and transmission will help to understand this behavior of silicon.
[1] B. Jaeggi, D.J. Förster, B. Neuenschwander, OSA Technical Digest, CLEO (2018), AM1M.3
[2] B. Jaeggi, D. J. Foerster, R. Weber, B. Neuenschwander, Adv. Opt. Techn. 7, 175 (2018)
[3] B. Neuenschwander, B. Jaeggi, D.J. Foerster, Th. Kramer, S. Remund, to be published in Proc. of ICALEO (2018)